Ethene (C.sub.2 H.sub.4), commonly known as ethylene, is a major starting material in the petrochemical industries. It is used to produce high- and low- density polyethylenes, vinyl chloride monomer, and ethylene oxide, among others. In 1989, worldwide ethylene production capacity was about 58 million tons a year. U.S. capacity was about 17 million tons annually.
Ethylene is commercially produced by the cracking of ethane, propane, and heavy hydrocarbons such as naphtha at very high temperatures. Using ethane as an example, EQU C.sub.2 H.sub.6 .fwdarw.C.sub.2 H.sub.4 +H.sub.2 ( 1)
The reaction is highly endothermic, about 143 Kjoules/g-mole at 725.degree. C. Steam is used to enhance cracking by reducing the partial pressure of the feedstock. Products are separated and recovered by cryogenic means. The whole operation is energy intensive. To achieve economy of scale, new plants are built with annual capacities of several hundred thousand tons each. A small improvement in ethylene production will have a great impact on the economics of the petrochemical industries.
Chloroethene, known as vinyl chloride monomer (VCM), is also a major commodity chemical in the world. It is used in the manufacture of polyvinyl chloride (PVC), one of the most important polymers. Commercially, two chemical reactions are used in the production of chloroethene, depending on the availability of the feedstock. The first reaction, used since the 1930's for large-scale production, involved acetylene and hydrogen chloride: EQU C.sub.2 H.sub.2 +HCl.fwdarw.C.sub.2 H.sub.3 Cl (2)
Between the 1930's and the 1950's, most of the acetylene feedstock for this method was obtained by the reaction between calcium carbide and water. Because of the high cost and scarcity of acetylene, this method has been replaced by a process using more readily available ethylene as feedstock.
The second reaction, presently used in chloroethene production worldwide, involves the pyrolysis of ethylene dichloride, C.sub.2 H.sub.4 Cl.sub.2 (EDC). EQU C.sub.2 H.sub.4 Cl.sub.2 .fwdarw.C.sub.2 H.sub.3 Cl+HCl (3)
Ethylene dichloride is produced in a balanced process by both oxychlorination and direct chlorination of ethylene. In direct chlorination, ethylene is reacted with chlorine gas to produce ethylene dichloride. In oxychlorination, HCl produced in EDC pyrolysis is reacted with ethylene and oxygen to produce additional EDC and the byproduct water. If acetylene is available, hydrogen chloride produced from EDC cracking can be used to react with acetylene to produce chloroethene. In recent years efforts has been made to develop a commercial process using unpurified acetylene from advanced ethylene plants operating under more severe conditions.
The primary disadvantage present in this balanced VCM process is the cost of ethylene, which is substantially higher than that of ethane. The vent gases released from the oxychlorination step also contain more than 100 ppm of vinyl chloride, which has been shown to be an atmospheric health hazard. To minimize undesirable VCM emissions from the oxychlorination step, many plants use pure oxygen instead of air. Use of pure oxygen also improves the utilization of ethylene and hydrogen chloride. However, oxygen plants are expensive. In addition, cracking of EDC induces coke formation. Milder cracking conditions to minimize coking limits the conversion of EDC. Plants using this process generate significant amounts of chlorinated hydrocarbon waste that must undergo disposal treatments.
Ethylene is produced in many plants in the U.S. Gulf coast using ethane and propane as feedstocks. Major efforts have been made to produce chloroethene from ethane directly, saving the cost of manufacturing ethylene. U.S. Pat. No. 3,923,913 disclosed a process of chlorinating ethane at high temperature to produce VCM, ethylene, and chloroethane. Ethylene and chloroethane are further oxychlorinated to produce additional VCM U.S. Pat. Nos. 3,629,354, 3,658,933, and 3,658,934 disclosed processes of chlorinating ethane to produce ethylene and hydrogen chloride, which were then processed by traditional oxychlorination to 1,2-dichloroethane and subsequent cracking to VCM. U.S. Pat. Nos. 3,796,641, 3,920,764, and 3,935,288 disclosed processes to produce VCM using ethane and chlorine in a two-reactor molten salt system. Undesirable byproducts are recycled to extinction. Because of poor selectivity in ethane chlorination, most of these disclosed processes require high recyle rates for the feedstock.
Propene(C.sub.3 H.sub.6), commonly known as propylene, is also a major starting material in the petrochemical industries. It is used to produce polypropylene, acrylonitrile, and propylene oxide, among others. In 1990, worldwide propylene production capacity was about 30 million tons a year. U.S. capacity was about 10 million tons annually.
Propylene is commercially produced by the steam cracking of propane or liquid hydrocarbons such as naphtha at very high temperatures. However, propylene is usually considered a valuable byproduct of ethylene production because propane is decomposed into ethylene as well as propylene. In propylene production from propane, EQU C.sub.3 H.sub.8 .fwdarw.C.sub.3 H.sub.6 +H.sub.2 ( 4)
The reaction is highly endothermic, about 129 K J/g-mole at 725.degree. C. Steam is used to enhance cracking by reducing the partial pressure of the feedstock. Propylene is also produced in refineries as a byproduct of the fluid-catalytic cracking and coking processes. It can also be produced as a main product by the thermal and catalytic dehydrogenation of propane. Both propane and propylene have very close boiling curves. In general, propylene is separated from propane and recovered by cryogenic means. Over 150 trays are used in a low temperature and high pressure distillation tower to recover polymer grade propylene. The whole operation is energy intensive.
Butene(C.sub.4 H.sub.8) is commonly known as butylene. Its main chemical use, which consumes only a small fraction of total available butylene, is for the production of butadiene. In the United States, a major portion of butylene is used as an alkylate feed. Outside the United States where liquid petroleum gas is not available, butylene is usually used as fuel. Butylene is commercially produced by the steam cracking, catalytic and thermal cracking, and dehydrogenation In steam cracking, and catalytic and thermal cracking, butylene is a byproduct of the ethylene production and refinery operation, respectively. In the dehydrogenation of butane, the conversion is limited by equilibrium consideration. EQU C.sub.4 H.sub.10 .fwdarw.C.sub.4 H.sub.8 +H.sub.2 ( 5)
The reaction is highly endothermic, about 120 K J/g-mole at 725.degree. C. In product recovery, the mixture of butane and butylene isomers, which have very close boiling curves, are separated by complicated methods. Again, the whole operation is energy intensive.
It is therefore an object of the present invention to provide a new method for the production of C.sub.2 H.sub.5 Cl, C.sub.2 H.sub.4 Cl.sub.2, ethylene, propylene, and chloroethene that minimizes or eliminates the disadvantages of the existing methods. Other objectives and advantages of the invention will become apparent from the following description and the accompanying drawings: